Influence of hypoxia on biogenic structure in sediments on the Louisiana continental shelf

As part of a study of the effects of seasonal hypoxia on sediment properties, samples were collected during the spring and late summer of 2009 from four sites of similar sediment type and water depth (30-39 m), but different recent history of bottom water oxygen concentration on the continental shelf of Louisiana. Sediment profile imaging (SPI), box coring, X-radiography, and computed tomography (CT) imaging were employed to characterize the biogenic structural differences in surficial sediments among a normoxic control site and three sites subjected to hypoxic events varying in frequency of occurrence. Results of the CT imagery indicated that macrobenthic biogenic structures were the most numerous at the H7 site that had experienced the least hypoxia in the past 23 years. The E4 site that had experienced hypoxia seasonally with a frequency between 50% and 75% of the time had the fewest biogenic structures in spring 2009, but exhibited recovery in terms of their abundance and diameter in summer 2009. E4 also exhibited high rates of bioturbation during the late-summer sampling as determined from excess 234Th. This suggests that the macrobenthos community at this site was in an active phase of recovery from hypoxia. At the A6 site, exposed to hypoxia with an annual frequency ≥75%, biogenic structures were numerous but dimensionally small, correlating with the average individual size of macrobenthos found there. The total volume maxima occupied by biogenic structures in the sediment occurred below the uppermost sediment intervals, with the exception of the spring sample from the D5 site that experienced hypoxia between 25% and 50% of the time. CT-imagery indicating effects of seasonal hypoxia on biogenic structure in the top 10 cm of sediments detected more structures than SPI, X-radiographic imagery, or macrobenthos census data. The presence of relict burrows probably inflated estimates of biogenic structures in the subcores, rendering an integrated result that included creation, destruction, and preservation of burrows and voids over time.